#app {
    .box3 {
        height: 300px;
        width: 300px;
        background-color: burlywood;
    }

    .box4 {
        width: 200px;
        height: 200px;
        background-image: url('./img/big.jpg');
        background-size: cover;
    }

    .box5 {
        width: 100px;
        height: 100px;
        background-image: url('./img/small.png');
        background-size: cover;
    }
}